The present invention relates to the labeling of audio signals to enable subsequent identification.
The present invention is particularly, but not solely, applicable to the labeling of audio and/or video sound track recordings such as to indicate the origins of the recordings, or the owner of the copyright in the recordings, or both. The labeling may also provide information as to payment of copyright royalties due.
European patent document EP-B-0245037 discloses and claims apparatus for the labelling of an audio signal, the apparatus comprising a plurality of filters to eliminate a plurality of specified frequency ranges from a given audio signal to form respective notches therein having respective center frequencies; code generating means to produce a code signal including an identifying portion and a message portion, the message portion formed of a plurality of bits, a first value of bits represented by a burst of a first respective specified frequency and a further value of bits being represented by a burst of a further respective specified frequency different from the first respective specified frequency, the specified frequencies selected to correspond to the respective center frequencies of the notches, combining means to sum the code signal with the audio signal containing notches; monitoring means to monitor the amplitude of the given audio signal; modulating means to set the code signal amplitude at a specified level below the given audio signal amplitude so that the code signal amplitude varies with the given audio signal amplitude; the apparatus characterized in the identifying portion of the code signal comprises a burst of both specified frequencies simultaneously and the apparatus further comprises frequency monitoring means to monitor the frequencies present in the given audio signal; and interrupting means to prevent the elimination of the plurality of specified frequency ranges and also prevent insertion of the code signal when the frequencies present in the given audio signal lie substantially outside a first given frequency range.
In earlier systems incorporating this apparatus, the code signal provided a label for the audio signal and usually consisted of two digital words, each word including an initial identifying portion of eight bits length comprising a burst of both frequencies. A data portion then followed comprising bursts of either the first or the second frequency to represent a xe2x80x9c1xe2x80x9d bit or a xe2x80x9c0xe2x80x9d bit. Two digital words were found necessary on account of the amount of data to be inserted to represent the International Standard Recording Code (ISRC). For stereophonic signals, the channel in which the code was inserted was changed from left to right alternately, so as to reduce the risk of detection of a code word by a listener to the program material.
Whilst the above system works perfectly well in practice, there is one specific application in which further improvement is desired. In this specific application, the labelled stereophonic channels are combined to give a monophonic signal before decoding (this is so that the same decoding apparatus can be used for both monophonic and stereo signals). In such an application, it becomes difficult to retrieve the coded signal, because the coded signal is normally inserted at an intensity related to the intensity of the program material in that particular channel. Thus with a combined signal, the coded signal will not necessarily be related to the intensity of the combined signal; thus it is more difficult to know at what level to expect to find the coded signal and this increases the difficulty of recovering the code. In addition, the code will be lost if only one channel of the stereophonic signal is received.
It has now been realized, in accordance with the invention, that it is not necessary to insert the code as code words introduced alternately in the two channels in order to prevent detection by a listener. In accordance with the invention, an entire coded label may be inserted into one channel without impairment of the audio signal.
Accordingly, the present invention provides in a first aspect apparatus for the labelling of a stereophonic audio signal, the apparatus comprising a plurality of notch filters having selected center frequencies to form notches at such selected frequencies in the channels of a stereophonic audio signal, code generating means to procure a coded label signal formed as one or more code words, the code being formed of selected signal bursts at the selected frequencies, and insertion means for inserting the coded label signal into both channels of the audio signal in said notches therein, with the code signal amplitude bearing a predetermined relationship to the audio signal amplitude of the respective channel.
Thus in accordance with this first aspect of the invention, since the entire label may be inserted into each channel of the stereophonic signal at a level related to the intensity/amplitude of the level of the audio signal, when the decoding operation takes place and the stereophonic channels are combined to give a monophonic signal, the coded signal will remain at a level related in a predetermined manner to the audio signal; thus the detection and decoding of the code label is facilitated.
Thus the present invention gives the advantage of better monophonic compatibility, as when the signal are combined to give a monophonic signal the level of the inserted code will track with the level of the monophonic signal. In addition the simultaneous labelling in a plurality of channels enables a reduction in the required amplitude of the coding signal in any given channel, which can further reduce audibility of the code. The invention also gives an unexpected benefit. In previous methods, the apparent position of the sound source of the code is always at one or other of the stereo loudspeakers, whereas in the present invention the code signal has an apparent position which coincides with the loudest program source for stereo signals, and this can move between the loudspeakers and is generally not in a fixed position. This can make the code even more difficult for a listener to detect in normal listening.
In a further aspect, the present invention provides apparatus for the labelling of an audio signal, the apparatus comprising a plurality of notch filters having selected center frequencies to form respective notches at such selected frequencies in stereophonic audio signal, code generating means to produce a coded label signal comprising one or more code words, the code being formed of selected bursts of the selected frequencies, and including insertion means for inserting at least part of the entire coded label signal simultaneously into each channel of the stereophonic audio signal in said notches therein.
The insertion means preferably includes means for detecting the intensity level of the audio signal at the frequencies at which the code label is to be inserted, and for preventing code insertion when the intensity of the audio signal is not sufficient to mask the code. In one preferred embodiment, the insertion means preferably includes means for assessing whether the residual audio signal remaining at the notch frequencies will interfere with code detection. In another preferred embodiment, a check is made prior to transmitting the coded audio signal on the code inserted at the notch frequencies, to assess whether the code can be decoded. This is preferably done by decoding the inserted code bit-by-bit prior to transmission.
In accordance with the invention, the label signal may comprise one or more data words. In situations where an ISRC code is to be inserted, two data words will usually be employed since one very long word carrying all the required information would increase the risk of detection by a listener. However in some applications where not so much data is required, a single code word may be sufficient.
A code word usually consists, as disclosed in European patent document EP-B-0245037 of an initial identifying portion comprising simultaneous bursts of both signal frequencies, followed by a message portion comprising bursts of either one frequency. In accordance with the invention, it has been found that an initial synchronizing portion is improved by providing it as a series of narrow pulses of predetermined width and spacing, within certain allowable deviations. The pulses can be used to derive a clock, which provides the starting point of the data, and the distance between data bits. This provides a significantly more complex signal requirement for the identification of the code, thereby reducing the likelihood of false data recovery and a significantly better signal from which to extract the data clock while minimizing the effects of noise on individual timing edges.
As preferred two notch frequencies are employed, with the notch frequency accurate to 1 Hz. The filters in one embodiment are 50 dB deep and 150 Hz wide at the 3 dB point. It will be understood for the purposes of this specification, that although a notch filter rejects a band of frequencies, this is so small in relation to the entire audio bandwidth that the filter can be represented by specifying a single frequency at the midpoint of the range.